1. Field of the Invention
The present invention relates to a knock control device of an internal combustion engine equipped with a knock control unit which calculates a background level based on a detected knock signal of the internal combustion engine and performs knock determination by comparing a variation of background level with a knock determination value.
2. Description of the Related Art
An engine etc. that runs on gasoline ignites and burns air-fuel mixture in a cylinder by a spark from an ignition plug during combustion stroke; however, when pressure in the cylinder is abnormally increased in the middle of flame propagation after ignition, a knock in which an unburned portion of the air-fuel mixture is self-ignited may generate before the flame propagation is completed. Then, a problem exists in that, when the knock is generated, vibration which gives a sense of discomfort to occupants is generated and, in the worst case, the upper surface of a piston is melted and damaged to break down the engine.
Consequently, there has been conventionally proposed knock control in which, when the knock is generated, ignition timing of an ignition plug is retarded to eliminate the knock and optimum torque and fuel consumption are achieved.
In the knock control, a vibration detection sensor so-called a knock sensor is equipped on a cylinder block in order to detect the generation of the knock and a vibration waveform of the engine, which is detected by the knock sensor, is analyzed to determine the presence or absence of the generation of the knock.
More specifically, a predetermined crank angle range after ignition, in which a vibration waveform can be obtained if the knock is generated, is regarded as a knock determination period; and an output signal from the knock sensor is analog/digital (A/D) converted in the knock determination period and a peak value is regarded as a peak hold value in the knock determination period. Then, a background level is calculated by performing smoothing processing of the peak hold value. Furthermore, the background level is performed as much as predetermined times (for example, two times) to set a knock determination value.
Then, the knock determination value is compared to the peak hold value; and when the peak hold value exceeds the knock determination value, a determination is made that knocking is generated and elimination operation of the knock is performed, for example, the ignition timing of the ignition plug is retarded.
In Patent Document 1, an upper limit value of updating quantity is increased in response to an increase of a variation of fuel injection quantity per a certain period or a variation of a throttle position while stabilizing by setting the upper limit value to the updating quantity of the background level; and accordingly, the background level is converged to the peak hold value immediately.
This object is to provide countermeasures against a phenomenon in that, when a load of the engine is increased, the peak hold value is increased in also the case where the knock is not generated, but if stabilization is continued by smoothing processing or limitation processing of updating quantity, the background level is not immediately increased; and as a result, a knock determination value becomes excessively small and therefore the knock is erroneously determined.
On the other hand, the knock may generate when the load of the engine is increased; and a very strong knock may continuously generate in some cases. In the case of such a state (hereinafter, referred to as a “continuous knock generation state”), ignition timing needs to be immediately retarded to eliminate the knock.
In Patent Document 1, as shown in FIG. 1, the background level is made to follow immediately when the load is changed; and therefore, the knock determination value is also increased immediately. As a result, even in the case of the very strong knock signal, determination cannot be made as to whether or not the knock is generated. Then, separation from the continuous knock generation state described above cannot be made and therefore the knock is continuously generated to cause a serious effect on the engine.
Furthermore, in order to solve the above mention, there is a method which achieves a balance between following capability at the time when the load is changed and detection of the continuous knock generation state by appropriately adjusting an upper limit value of updating quantity to properly follow the background level as shown in FIG. 2. However, in the case where a load of the engine is gradually increased so that a change in the background level is below the upper limit value of updating quantity, the knock cannot be detected as shown in FIG. 3 and separation from the continuous knock generation state cannot be made.
That is, the method which forms the knock determination value based on the peak hold value and detects the knock by comparing with the peak hold value is effective at the time when the load is transited; however, the continuous knock generation state cannot be detected completely by only this method. Therefore, a knock detection method using the knock determination value which does not depend on the peak hold value is needed. If this method is referred to as a knock detection method during stationary time in comparison with the aforementioned method which depends on the peak hold value, the following Patent Document 2 to Patent Document 4 are conventionally proposed as knock detection techniques during such stationary time.
In Patent Document 2, an inclination of an envelope of a knock signal is detected by envelope detection to detect the knock using the difference between an inclination at the time when the knock is generated and an inclination other than that.
In Patent Document 3, a knock is detected by making it a condition that a knock signal is not lower than a reference value and an average inclination is not lower than a reference value.
In Patent Document 4, a knock is detected by regarding multiplication of a peak hold value by a coefficient as a knock determination value, the peak hold value being within a predetermined period at which a knock is not generated before ignition, and the coefficient being defined by engine speed and load.